There are a number of great planetariums in the Tokyo area, and one of them is Cosmo Planetarium Shibuya. Located just a short walk from Shibuya Station, the venue hosts a variety of themed programs relating to our galaxy, including material geared towards both adults and children.
The projection schedule can be found on the planetarium's official website, and at the time of writing there are several programs available. A children's program is available on weekends and includes a cartoon representation of Hachiko teaching children about our universe, answering questions like "how big is the universe?" and "what types of stars are there?". This program runs until July 10th, but should be succeeded by another children's themed program.
Other programs available at present include "Starry Night", which pairs relaxing music with projections of starry skies in Hokkaido and Okinawa, and a more energetic program combining music from Japanese rock band Acidman with glimpses of our galaxy. You can see a preview of the Acidman program in the video below.
Something to keep in mind if you're planning a visit here is that the programs are available in Japanese only. Even without Japanese language comprehension there is still plenty to enjoy, but expectation management is important.
Admission to Cosmo Planetarium Shibuya is priced at a reasonable ¥ 600 for adults, and ¥ 300 for elementary and junior high school aged students. Children under elementary school age receive free admission.